Marked up abstracts sometimes inexplicably return to be marked "Undecided" although I just marked "Included" or "Excluded"

It might appear that the excludes or includes are lost but rest assured all actions you do are saved unless you receive a server error message. The reason why you don't see them immediately is that it takes typically 10-20 seconds for the labels to load from the server again. This can happen especially in the case where you include/exclude a large number of references at once. Try to do it in smaller batches instead and give them time to reflect on the left panel.